Solar collectors having a high energy efficiency absorb the solar radiation falling in thereon, transform it to heat energy, and deliver the thermal energy to a heat exchanging medium with least possible losses.
The most critical part of a solar collector having high energy efficiency is its absorbing surface, which has to be spectral selective, i.e. have the ability of absorbing radiation in the wave length range of 0.3 to 2.5 micrometer where the substantial part of the solar radiation energy is present. It shall further posses a low thermal emittance in the wave length range of 4.0 to 50 micrometer.
A number of methods are known to obtain reflecting surfaces on e.g. A solar collector, viz., anodization with a subsequent metal pigmentation and sputtering or atomizing.
Sputtering is obtained by means of having the cathode bombed by ions in a plasma gas formed by a potential difference between the cathode and the receiving material, whereby the cathode material is being atomized and the material forms a coating upon the receiving material. The process takes place in a vacuum chamber.
Solar energy absorbing means are known from SE-C-7713121-7, which means comprises a sheet formed element of aluminum sheet and a surface layer applied upon this by means of anodizing, surface layer has a high spectral selectivity and which consists of an aluminum oxide layer having been tightened afterwards and having a maximum thickness of 1.5 micrometer. The surface layer comprises small pores comprising metal, which layer is completely included into the oxide layer and are isolated from each other. The manufacture of the absorbing surface takes place using anodization and introduction of metal into the pores in an electrolytic way. This method is carried out in two different process steps and two different metals are used. The methods are labour intense and requires a lot of man power. After the electrolytic treatment a number of environmentally dangerous residual products are produced which have to be safely stored and destructively treated.
Further, from U.S. Pat. No. 4,964,963 there is known a method using two process steps, whereby a solar energy reflector is primarily coated with a stainless steel in an oxidizing atmosphere by atomizing whereupon a second metal layer having a high reflecting ability is atomized above the steel.
From U.S. Pat. No. 4,990,234 and the international application WO 87/06626 there is known different methods for coating glass surfaces with another material, by means of the atomizing technology.
DE-3,022,714-C2 describes a method for the preparation of a selectively absorbing layer for a solar collector using an argon-nitrogen sputtering atmosphere, where nickel is transferred to nickel nitride by means of cathode atomizing. The process provides a surface having high absorption values and a relatively low thermal emittance, but it is not suited for industrial manufacturing as the treatment time (30 min for a treatment surface of 1.6 m.sup.2) during the atomizing is far too long.